First commercially deployed in 1998 by TruePosition in Houston, Tex., overlay network-based wireless location systems have been widely deployed in support of location-based services including emergency services location. Network-based systems rely on the reception of the wireless device originated uplink mobile transmission, which is used in a time-of-arrival (TOA), time-difference-of-arrival (TDOA), power-of-arrival (POA), power-difference-of-arrival (PDOA) or angle-of-arrival (AoA) location calculation. Network-based location calculations can be combined with mobile-based measurements, collateral information, or with other network-based location calculations to form hybrid locations.
As realized and noted in extensive prior art, the ability to routinely, reliably, and rapidly locate cellular wireless communications devices has the potential to provide significant public benefit in public safety and convenience and in commercial productivity.
A cellular network is designed to exploit frequency reuse. That is, careful planning and surveying of radio transmission frequencies to control adjacent and co-channel interference is performed in a cellular network on a more-or-less continuous basis. In addition to frequency planning, the various wireless communications protocols were also designed to both minimize and tolerate adjacent and co-channel interference. Techniques for minimizing such interference include SAT tones in AMPS, color-codes in IS-136, frequency hopping patterns in GSM, code separation in CDMA (IS-95 & IS-2000) and UMTS (also known as W-CDMA), and both frequency hopping patterns and zero-autocorrelation Zadoff-Chu sequences in LTE.
Since a network-based WLS relies on receiving the signal from the mobile-of-interest at geographically distributed land-based receivers, the likelihood of co-channel interference is increased since the geographic reuse pattern of the underlying cellular network cannot be relied on to isolate transmissions.
As detailed in the U.S. Pat. No. 5,327,144, “Cellular telephone location system,” and U.S. Pat. No. 6,047,192, “Robust, efficient, localization system”, correlation processing of signals received by geographically separated wireless receivers can be used with extremely weak signals such as those found in frequency reuse cellular systems to generate time-difference-of-arrival (TDOA), angle-of-arrival (AoA), and hybrid TDOA/AoA location and velocity estimates.
The inventive techniques and concepts described herein apply to time and frequency division multiplexed (TDMA/FDMA) radio communications systems including the widely used IS-136 (TDMA), GSM, and OFDM wireless systems, as well as the OFDM-based WIMAN (IEEE-802.16), WiMAX (IEEE-802.20), and Long Term Evolution (LTE) Evolved Universal Terrestrial Radio Access Network (EuTRAN). The Global System for Mobile Communications (GSM) model discussed above is an exemplary but not exclusive environment in which the present invention may be used.
The following U.S. patents describe systems and methods of interference cancellation and antennae selection in association with a wireless location system: U.S. Pat. No. 6,765,531 B2, Jul. 20, 2004, “System and Method for Interference Cancellation in a Location Calculation, for Use in a Wireless Location System”; and U.S. Pat. No. 6,661,379, Dec. 9, 2003, “Antenna Selection Method and System for a Wireless Location System”. These provide further background information relating to the presently described subject matter.